Harvesters such as windrowers, tractors, and hay foresters have to balance functionality with weight distribution. On one hand, the harvester needs to include a hitch at the front of the vehicle so that it can attach a header or some other harvesting equipment. On the other hand, the harvesting equipment is very heavy when attached on the front of the vehicle. If the vehicle is operating with a header attachment, the shock absorbers must accommodate weight distributed on the front end of the vehicle. If the header is not attached to the vehicle, the weight distribution of the vehicle is altered dramatically on most current vehicle designs such that the front of a harvester may be elevated over a horizontal or substantially horizontal plane. This elevation makes it difficult to attach a header. The weight distribution to the rear of the vehicle can also cause vibration and shimmying across the rear axle or axels during a high-speed, or non-harvesting mode. Such vibration or shimmying can put undue stress on the rear suspension system.
Typical construction for such vehicles include front ground wheels mounted on the frame at fixed angles parallel to each other and parallel to a center line of the frame and rear ground wheels mounted on a respective castor. Each of the front ground wheels is typically driven by a respective drive motor which allows variable speed in both the first and second directions such that steering of the tractor is effected by a differential in speed between the front wheels with the rear wheels following the steering in a castoring action. A pair of damper cylinders are each connected between a bracket on a rear axle of the frame and a lever in the castor plane of the castor so as to provide a damping force tending to restrict rotation of the respective second wheel about the respective vertical pivot axis with the damping force at a maximum value when the rear wheels are parallel to the center line in either the forward or reverse directions.
Such hydrostatically driven vehicles used primarily for swathing are commonly used and readily available. The vehicles typically carry at a forward end on suitable sprung supports a header for cutting standing crop with the crop being transported on the header to a suitable discharge location generally centrally of the vehicle for forming a swath in the field from the cut crop.
Such vehicles generally include a pair of front wheels just behind the header which are fixed to the frame of the vehicles so that they remain at an angle parallel to each other and parallel to a center line of the tractor. The tractor is supported at the rear end by a pair of castor wheels which are attached to a cross axle pivotally connected to the frame at a center horizontal pivot point, commonly known as a walking beam. The axle is typically supported relative to the ground the castor wheels at the ends of the rear axle so that the wheels trail behind a vertical pivot mount for the wheels.
The front wheels only are driven and are driven in a manner which allows full control in the forward and reverse directions so that steering is effected by a differential speed between the two front wheels with the rear wheels following in the conventional castoring action.
It is also known that such vehicles can travel more effectively at high speed when reversed in direction so that the driven wheels are at the rear and the castor wheels at the front. The castor wheels of course rotate through 180° to trail behind the vertical pivot which is now forward of the castor wheels as the tractor moves at relatively high speed in this reversed direction.
Castor shimmy is an inherent problem on vehicles of the above type. Recent machines have been increased in speed up to a maximum road speed of 24 mph instead of 16 which is the standard in the industry. Speed aggravates the shimmy problem.
Loss of ground contact (going over bumps) also aggravates the problem and as a result weight is often added to prevent this. This results in increased power requirements and increased fuel consumption.
Many such vehicles use as the rear suspension of the rear castor wheels the simple pivoting rear axle or walking beam and while speeds have been relatively low, this has been acceptable. Of course the tires themselves also provide flexure which contributes to the suspension. However the use of only the tires as a suspension in rough terrain can cause sever pitching leading to significant fore and aft acceleration of the operator station.